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Aquarium Displacement Calculator

Your tank holds less water than its dimensions suggest. Add the substrate, rock, equipment and the air gap at the top to find the volume that actually circulates, which is the number dosing and water changes should use.

Step 1

Tank volume

Calculated volume: . For non-rectangular tanks, calculate with the shape calculator and enter the gallons directly.

Step 2

Displacement sources

Displaces (solid fraction only; the bed's voids stay water-filled)

Displaces

Displaces

Removes

Substrate displacement by depth and tank

Solid volume of a coarse gravel bed (38% voids) at common depths.

Substrate depth10 gal (20×10 in)29 gal (30×12 in)55 gal (48×13 in)75 gal (48×18 in)
1 inch 0.54 gal0.97 gal1.67 gal2.32 gal
2 inches 1.07 gal1.93 gal3.35 gal4.64 gal
3 inches 1.61 gal2.90 gal5.02 gal6.96 gal

Why displacement matters

Dosing instructions assume the volume of water actually in the system. A 75-gallon reef with 80 pounds of rock, a two-inch sand bed and an inch of air at the top has closer to 62 gallons of water. Dosing for 75 overshoots by a fifth, every time. The same gap affects water change percentages, medication concentrations and how much salt to mix.

Substrate is the subtle one. A gravel bed is not solid: roughly 38 percent of it is water-filled voids. This calculator removes only the solid fraction, which is why a two-inch bed displaces less than a plain length × width × 2 calculation suggests. Sand has fewer voids than gravel, and planted soils more.

Frequently asked questions

Porous live rock displaces about 0.022 gallons per pound, so 50 pounds removes roughly 1.1 gallons. Dense dry rock displaces more per pound, about 0.03 gallons.
Because gravel is porous. Water fills the gaps between grains, so only the solid 62 percent of the bed actually displaces water.
No, external filters add water to the system rather than displacing it. Add their volume with the sump calculator instead.

Related tools and references

All shape calculatorsTwenty tank shapes in gallons and litres. Unit ConverterConvert between US gallons, litres, imperial gallons and cubic units. Displacement CalculatorSubtract substrate, rock, equipment and fill gap to find true water volume. Sump Volume CalculatorAdd display, sump and refugium for total system volume. Volume formulasEvery formula with worked examples. How to measureInside dimensions and fill height done right.